The invention relates generally to high temperature polymers and in particular to phthalonitrile polymers and their cure.
There is an extensive range of applications for phthalonitrile resins including as matrix materials for lightweight, high temperature resistant carbon fiber composites for use in aircraft engine nacelles, as binding filler suitable for use in clutch or brake linings, and in hot molds as casting material.
Interest in fiber-reinforced composites for advanced aerospace applications has led to the search for high temperature polymers that are easily processed and exhibit high thermal and oxidative stability. Epoxies and polyimides are now being used but each has its disadvantages. Conventional epoxy-based composites and adhesives have a 200.degree. C. maximum service limit and polyamide resins used in composites matrices have a 300.degree. C. maximum service limit.
A major problem of the polyamide system is the inability to process void-free and blister-free components in high yield because of the evolution of volatile components formed during the polymerization condensation reaction. Other problems associated with both polyamide and epoxies include their brittleness, water absorptivity and engineering reliability.
Phthalonitrile resins are proving to be superior in physical and chemical properties to epoxies, polyimides and other plastics as matrices for fiber-reinforced composites and in other applications. A major advantage of phthalonitrile resins, compared to other plastics, is their ability to withstand temperatures in excess of 300.degree. C. for extended periods without permanent damage to the coatings, plastics or composites made therefrom. Such resins usually contain a substantial proportion of aromatic structures, but cured polymers composed solely of aromatic rings tend to be brittle and intractable. A resin having flexible linkages between the aromatic rings minimizes or greatly reduces brittleness and intractability. Polyphthalonitrile resins with diether linkages are materials which meet these goals.
To achieve the objective of developing polymeric materials that are easily processable and exhibit superior thermooxidative properties relative to current commercial polymers, phthalonitrile monomers containing aryl, ether, imide, sulfonyl, thioether, and fluorine moieties have been synthesized and polymerized. Neat polymerization of these monomers, which do not contain active hydrogen atoms, is extremely difficult and requires several days of continuous heating at 260.degree.-300.degree. C. before a viscosity increase becomes evident.